Linux 5.15 GKI for Samsung Galaxy Tab A8 10.5 (SM-X200)

Linux 5.15.178 GKI bring-up tree for Samsung Galaxy Tab A8 10.5 (2021) with Unisoc Tiger T618 (ums512_25c10 / sharkl5pro).

Status: Early Bring-Up / Not Booting to Userspace Yet (WIP) The kernel initializes core subsystems and executes /init, but halts during userspace handoff.

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🐛 Documented Bugs & Issues to Fix

1. Cortex-A75 CPU7 / Stop Machine Soft Lockup

• Symptom: At late boot (~50s), watchdog reports soft lockup across all online cores: watchdog: BUG: soft lockup - CPU#X stuck for 48s! [migration/X] <- multi_cpu_stop <- stop_machine_cpuslocked.
• Root Cause: When Android init or CPU hotplug invokes stop_machine_cpuslocked (or CPU takedown), all online CPUs wait in multi_cpu_stop for an atomic acknowledgement (thread_ack). CPU 7 (big core) does not process the cross-core stop IPI, causing CPUs 0–6 to spin indefinitely.
• Current Workaround: Disabled CPU7 from cpu_possible_mask in arch/arm64/kernel/smp.c (set_cpu_possible(7, false)). Needs proper PSCI / GICv3 interrupt affinity investigation.

2. Display Freeze / SimpleFB & Panel Handoff

• Symptom: Screen stays frozen on the Samsung bootloader splash logo; does not transition to Android boot animation.
• Root Cause: The bootloader leaves the framebuffer at physical address 0x9e000000 (1200x1920, 32-bit). simplefb and the Unisoc DPU/DSI DRM driver (sprd_disp) have not completed panel power-up and timing handoff for the Novatek lcd_nt36523b_qc_inx_mipi_hdp panel.
• Next Step: Port panel power sequencing and DSI initialization sequences from Samsung 4.14 kernel DTS.

3. Userspace Init Crash & Reboot to Download Mode (Odin)

• Symptom: Android init runs for ~36 seconds, then executes reboot: Restarting system with command 'shell', which triggers Samsung Download (Odin) Mode.
• Root Cause:
  1. GSI HALs fail to communicate with vendor services (Trusty TEE / skeymaster4dev reports error: -49 due to missing tsupplicant).
  2. Successive boot failures trigger Samsung's hardware watchdog and write DLOW (PARAM_BOOT_DOWNLOAD_FAIL) to the param partition (offset 0x900000).

4. Boot Image DTB Format Requirements

• Symptom: Immediate drop to Odin mode with black screen upon flashing kernel.
• Root Cause: Unisoc LK bootloader requires the DTB section in boot.img (header v2) to be an Android DT Table (0xd7b7ab1e magic). A raw FDT blob (0xd00dfeef) triggers an assertion in LK (fdt_chosen_bootargs_append / overflow).
• Solution: Repack using magiskboot repack to preserve header checksums, offsets, and the DT table structure.

5. eMMC SDHCI Compatibility

• Status: Patched in drivers/mmc/host/sdhci-sprd.c to match DT compatible sprd,sdhc-r11p1, add clock name fallbacks (sdio_clk), and make state_uhs pinctrl non-fatal. Needs validation under sustained I/O.

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🛠️ Build

export ARCH=arm64
export LLVM=1
make gta8_defconfig
make -j$(nproc) Image dtbs